1. Field of the Invention
The present invention relates to an image-receiving sheet for thermal transfer printing which is provided with a layer containing aggregates of fine particles of a thermoplastic resin. More particularly, the present invention relates to an improvement in an image-receiving sheet for thermal transfer printing comprising an intermediate layer and an image-receiving layer being disposed in sequence on a substrate, in which at least either said intermediate layer or said image-receiving layer contains aggregates of fine particles of a thermoplastic resin.
2. Related Background Art
Thermal printing systems in which printed images are obtained upon reception of input signals are made up of a relatively simple apparatus and are inexpensive and low in noise. In view of this, they are increasingly utilized in various fields such as facsimiles, terminal printers for electronic computers, printers for measuring instruments, and the like.
As the recording medium to be used in these thermal printing systems, there has been generally used a so-called spontaneous developing heat sensitive paper having a recording layer capable of causing a physical or chemical change upon application of heat to provide color development. However, the spontaneous developing heat sensitive paper of the color developing type has disadvantages such that it is liable to undesirably cause color development during the fabrication process or during storage; and the images printed on the paper are poor in storage stability and they are apt to fade on contact with organic solvents or chemicals.
In order to improve the above situation, there has been proposed a printing system wherein a recording medium in which a coloring material such as a dye or colorant is utilized is used instead of the foregoing spontaneous developing heat sensitive paper, for example, as disclosed in Japanese Unexamined Patent Publication Sho. 51(1976)-15446. In the printing system disclosed in this publication, a colorant transfer sheet comprising a substrate such as a paper or polymer film and a colorant layer containing a coloring material (which is in a solid or semi-solid state at ordinally temperature) being formed on the substrate is firstly provided, this colorant transfer sheet and a image-receiving sheet are superposed so as to make the colorant layer of the colorant transfer sheet contacted with the image-receiving sheet, and the laminate is heated from the non-faced side of the former sheet by a heating means such as a thermal head to transfer the coloring material in the colorant layer of the colorant transfer sheet onto the image-receiving sheet upon application of electric signals provided by the heating means, whereby images corresponding to image information are recorded on the image-receiving sheet. In this printing system, the coloring material in the colorant layer of the colorant transfer sheet is caused to melt, evaporate or sublimate by the application of heat and transferred onto the image-receiving sheet, thereby forming a record image through adhesion, adsorption or reception of the coloring material on the image-receiving sheet. In view of this, this printing system has been evaluated as being advantageous from the viewpoint that there can be used an ordinary paper (wood free paper) as the image-receiving sheet. Further, as for this printing system, when a sublimable dye is used as the coloring material, there can be obtained a printed image excelling in tone reproduction. In order to develop this advantage in the full-color printing, various studies have been made.
However, there are disadvantages for the above printing system wherein an ordinary paper (wood free paper) is used as the image-receiving sheet such that sufficient dye-reception hardly occurs causing printed images which are poor in color density (optical density) and whose image is markedly discolored as time lapses.
In order to eliminate these problems, there has been proposed the use of an image-receiving sheet having an image-receiving layer containing a thermoplastic resin as the main constituent which is formed on a substrate as disclosed in Japanese Laid-open Patent Publication Sho. 57(1982)-107885, Japanese Laid-open Patent Publication Sho.60(1985)-38192 or U.S. Pat. No. 3,601,484.
The use of this image-receiving sheet is effective in somewhat improving the recording sensitivity and storageability. However, there still remains a problem in that when an ordinary paper (wood free paper) is used as the substrate, it is difficult to provide a desirable image-receiving layer containing a thermoplastic resin as the main constituent of uniform thickness with the paper and thus, the resulting image-receiving sheet unavoidably becomes such that is poor in recording sensitivity and provides undesirable images inferior in quality. In order to eliminate this problem, there has been proposed another image-receiving sheet having an intermediate layer comprising a thermoplastic resin which is disposed between a substrate and an image-receiving layer as disclosed in Japanese Laid-open Patent Publication Sho. 60(1985)-236794 or Japanese Laid-open Patent Publication Sho.61(1986)-144394. The image-receiving sheets disclosed in these publications are so designed that their image-receiving layer can be effectively contacted with the colorant layer of the colorant transfer sheet to prevent occurrence of negative phenomena such as air-gap upon printing and that they can exhibit an improved recording sensitivity and provide improved record images.
In addition, aiming at improving the quality of an image recorded, there has been proposed a further image-receiving sheet having an intermediate layer containing fine particles of a specific resin which is disposed between a substrate and an image-receiving layer as disclosed in Japanese Laid-open Patent Publication Sho.63(1988)-87286, Japanese Laid-open Patent Publication Sho.64(1989)-27996, Japanese Laid-open Patent Publication Hei.1(1989)-136784, and Japanese Laid-open Patent Publication Hei.2(1990)-139293. The image-receiving sheets disclosed in these publications are effective especially in obtaining desirable recorded images free of missing transfer portion. However, any of these image-receiving sheets is still accompanied with a disadvantage in that desirably uniform record images are hardly provided because the resin fine particles contained in the intermediate layer are undesirably varied.
In order to attain a desirable recording sensitivity for the image-receiving sheet resulting in providing a high quality recorded image while eliminating the above problem, there has been proposed to use synthetic papers or films as the substrate of the image-receiving sheet. This proposal is, however, not sufficient enough to solve the problem not only because the image-receiving sheet the substrate of which comprising such synthetic paper or film is liable to deform by heat applied upon recording but also because such synthetic paper and film are costly.